Add methods to enable configuration of ResourceConstraints based on limits derived at runtime.

src/platform-linux.cc

Index: src/platform-linux.cc
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index 74d473f4a125091038a48981037db56900676940..0c910638c1f077f94cd415bda61fefe6b0437bdb 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -313,6 +313,17 @@ void OS::SignalCodeMovingGC() {
 }
 
 
+uintptr_t OS::TotalPhysicalMemory() {
+  intptr_t pages = sysconf(_SC_PHYS_PAGES);
+  intptr_t page_size = sysconf(_SC_PAGESIZE);
+  if (pages == -1 || page_size == -1) {
+    UNREACHABLE();
+    return 0;
+  }
+  return static_cast<uintptr_t>(pages) * page_size;
+}
+
+
 // Constants used for mmap.
 static const int kMmapFd = -1;
 static const int kMmapFdOffset = 0;